Xerographic and electrophotographic printing and marking engines schedule control patches for calibration and other machine diagnostic procedure. The control patches are printed between images in what is called “inter-document zones” (IDZ) on the photoreceptor belt and/or other image transfer member using a calibration procedure having a desired toner area coverage, for example, as disclosed in U.S. Pat. No. 6,016,204, which is incorporated by reference herein in its entirety. The areas of the image transfer member where images of the print job are printed, by contrast, are called the “customer image zones,” or “image zones.”
The control patches may include one or more toner density patches. The toner area coverage, AC is defined as the percentage of toner area covering a unit halftone cell in a sample target that is available to reflect. As known in the art, control patches may be varied uniformly for each test patch from 0 to 100%. These control patches are sensed and machine parameters may be adjusted to maintain a tone reproduction curve (TRC).
As the marking engine's performance degrades over time, different image quality (IQ) problems may arise. In particular, one IQ problem of concern is reload. Reload defects may occur when the reload performance of the developer apparatus degrades to the point where the customer image impacts the control patches, and/or, the control patches impacts the customer image.
FIG. 1 illustrates a known developer apparatus 100 in an electrophotographic printing system. Such a system is disclosed, for example, in U.S. Patent Application Publication No. 2006/0109487, which is incorporated by reference herein in its entirety.
The apparatus 100 includes a reservoir 164 containing developer material 166. The reservoir includes augers, indicated at 168, which are rotatably-mounted in the reservoir chamber. Augers 168 serve to transport and to agitate the material within the reservoir and encourage the toner particles to charge and adhere triboelectrically to the carrier granules. Magnetic brush roll 170 transports developer material 166 from the reservoir to loading nips 172, 174 of donor rolls 176, 178. Metering blade 180 removes excess developer material from the magnetic brush roll and ensures an even depth of coverage with developer material before arrival at the first donor roll loading nip 172.
At each of the donor roll loading nips 172,174, toner particles are transferred from the magnetic brush roll 170 to the respective donor roll 176,178. The carrier granules and any toner particles that remain on the magnetic brush roll 170 are returned to the reservoir 164 as the magnetic brush continues to rotate. The relative amounts of toner transferred from the magnetic roll 170 to the donor rolls 176, 178 can be adjusted, for example by: applying different bias voltages to the donor rolls; adjusting the magnetic to donor roll spacing; adjusting the strength and shape of the magnetic field at the loading nips and/or adjusting the speeds of the donor rolls.
Each donor roll transports the toner to a respective development zone 182,184 through which the photoconductive belt 10 passes. At each of the development zones 182, 184, toner is transferred from the respective donor roll 176, 178 to the latent image on the belt 10 to form a toner powder image on the latter.
In the device of FIG. 1, each of the development zones 182, 184 is shown as having a pair of electrode wires 186, 188 disposed in the space between each donor roll 176, 178 and belt 10. The electrode wires may be made from thin (for example, 50 to 100 micron diameter) stainless steel wires closely spaced from the respective donor roll. The wires are self-spaced from the donor rolls by the thickness of the toner on the donor rolls and may be within the range from about 5 micron to about 20 micron (typically about 10 micron) or the thickness of the toner layer on the donor roll.
For each of the donor rolls 176 and 178, the respective electrode wires 186 and 188 extend in a direction substantially parallel to the longitudinal axis of the donor roll. An alternating electrical bias is applied to the electrode wires by an AC voltage source 190. The applied AC establishes an alternating electrostatic field between each pair of wires and the respective donor roll, which is effective in detaching toner from the surface of the donor roll and forming a toner cloud about the wires, the height of the cloud being such as not to be substantially in contact with belt 10.
After development, excess toner may be stripped from donor rolls 176 and 178 by respective cleaning blades (not shown) so that the magnetic brush roll 170 meters fresh toner to the clean donor rolls. As successive electrostatic latent images are developed, the toner particles within the developer material 166 are depleted. A developer dispenser 105 stores a supply of toner particles, with or without carrier particles. The dispenser 105 is in communication with reservoir 164 and, as the concentration of toner particles in the developer material is decreased (or as carrier particles are removed from the reservoir as in a “trickle-through” system or in a material purge operation as discussed below), fresh material (toner and/or carrier) is furnished to the developer material 166 in the reservoir. Developer housing 164 may also include an outlet 195 for removing developer material from the housing in accordance with a developer material purge operation as discussed in detail below. Outlet 195 may further include a regulator (not shown) such as an auger or roller to assist in removing material from the housing.
Each donor roll rotates and when it completes a full rotation, the donor roll has toner with different charge/mass ratio than regions where the toner has been on the roll for multiple revolutions. In particular, the toner on the donor roller may be less in regions of the donor roll where toner was removed during previous revolutions. This leads to the possibility of a reload defect, which appears as a lighter area in the subsequent regions. As a result, a “ghost” image of a previous control patch may be printed with a customer image or vice versa.
FIG. 2 illustrates a plot of the toner mass on a region of a donor roll in the developer apparatus immediately after printing. As the donor roll continues to rotate more and more toner will accumulate on the donor roll, thereby replenishing the toner mass on the donor roll. This process may take multiple revolutions of the donor roll. After a sufficient number of rotations, the toner mass at that region of the donor roll will be completely replenished (mass Mo).
A problem arises, however, where an image to be printed requires more toner (mass M1) than that region of the donor roll might be presently able to provide (mass M2), i.e., before that region of the donor roll has been replenished with toner. If so, there may be the possibility of a reload artifact appearing in the printed document.
One possible solution is to image control patches in an edge zone on the photoreceptor belt to ensure that the control patches do not interfere with the customer print zone. Such a solution was disclosed, for example in U.S. patent application Ser. No. 11/931,721 filed Oct. 31, 2007, herein incorporated by reference in its entirety. However, for some print systems this may not be feasible.